RF: apply_read_scaling fix for integer slope/inter

``apply_read_scaling`` should be able to cope with integer slope and
intercept values, although we aren't using these in the code-base yet.
TST: add arrayproxy API validators

Add API validators for the basic arrayproxy uses (Analyze types, MGH
format).
RF: proxy uses header fields, deprecate header copy

Plan not to copy header for the array proxy. Copy out the relevant
header fields and do scaling in the proxy.